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Generator & Backup Power Calculators

A generator decision is really five decisions: how many watts you need, how big a set that makes, what it costs installed, how much fuel it drinks, and whether the numbers on the label mean what you think. These six calculators cover all of it, in US or metric units.

About these generators calculators

Almost every generator bought too small was sized on running watts. A refrigerator that draws 700 watts steadily demands about 2,200 for the half-second its compressor spins up, and a generator that cannot supply that surge stalls, trips, or slowly cooks its own alternator. Almost every generator bought too large was sized by adding every surge together — which never happens, because motors do not start in unison. The honest number sits between the two: every running watt, plus the single largest starting surge, plus headroom.

The second thing people get wrong is the unit. Generators are advertised in watts in North America and in kVA across most of Europe, and the two are not interchangeable. kVA is apparent power — volts times amps, before power factor is applied. A 5 kVA set at the usual 0.8 power factor delivers 4,000 watts, not 5,000. Motors add a third unit, horsepower, and a fourth complication, efficiency, because the electrical input to a motor is always larger than its mechanical output. The kVA calculator converts between all four in one place.

Then there is money, which splits in two. The purchase is the smaller half of a standby installation: the pad, the gas line, the transfer switch, the electrical work and the permit routinely cost as much as the generator itself, and none of it appears on the price tag. Running cost is the other half, and it is a function of load rather than size. A generator at quarter load burns far more fuel per kilowatt-hour than the same generator at three-quarters, which is why an oversized set is expensive twice over.

Two cautions apply to everything in this cluster. Never backfeed a generator into a socket — it energises the utility line outside and can kill the people repairing it; anything connected to house circuits needs a transfer switch or interlock fitted by a licensed electrician. And never run an engine indoors, in a garage, or near an open window. Carbon monoxide from a portable generator is the leading cause of death after a storm, and it kills faster than most people believe.

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